This invention relates to a control system of an alternating-current motor, and more particularly to a positioning control system in which an alternating-current motor is the actuator.
Speed control systems of an alternating-current motor in which the frequency of the motor power supply is controlled, have been, heretofore well known. For example, the heretofore known variable frequency induction motor drive systems are summarized in a paper by Jalat T. Salihi on "Simulation of controlled-slip variable-speed induction motor drive systems" in IEEE transactions in industry and general applications, Vol. IGA-5, No. 2, March/April 1969. And heretofore known variable frequency synchronous motor drive systems are described in a paper by Armando Bellini, Alessandro De Carli, and Mario Murgo on "Speed control of synchronous machines" in IEEE transactions on industry and general applications, Vol. IGA-7, No. 3, May/June 1971.
In all these prior disclosures, there is not a description of an automatic control system of an alternating-current motor. In an automatic control system of an alternating-current motor, there must be provided a control variable limiting means which receives the initial value for the control variable as synthesized from the reference signal and the controlled variable, and which automatically limits the variable range of the control variable within a range adapted for the control purpose.
In all heretofore known variable frequency alternating-current motor drive systems, this control variable limiting means which automatically limits the control variable within a range adapted for the control purpose has not been found.
In U.S. patent application Ser. No. 543,026, now U.S. Pat. No. 4,009,427, entitled "Adaptive control system for an alternating-current motor" which was filed on Jan. 22, 1975 by the inventor of the present invention and which will hereafter be referred to as the inventor's prior patent application, an alternating-current motor drive system which is adapted use in a positioning control system is disclosed.
In this control system disclosed by the inventor's prior patent application, the control variable is the slip frequency f.sub.s for an induction motor or the load angle .theta..sub.d (other names for the load angle are displacement angle, torque angle, coupling angle, internal angle; the term of load angle will hereafter be used) for a synchronous motor. The maximum magnitude .theta..sub.k of the load angle .theta..sub.d or the maximum magnitude f.sub.k of the slip frequency f.sub.s is predetermined, and a limiting means automatically limits the magnitude of .theta..sub.d within the value of .theta..sub.k, or the magnitude of f.sub.s within the value of f.sub.k.
In the inventor's prior patent application, it is disclosed that the magnitude f.sub.T of the slip frequency at which the maximum magnitude of the torque is developed in an induction motor or the magnitude .theta..sub.T of the load angle .theta..sub.d at which the maximum magnitude of the torque is developed in a synchronous motor is approximated by a constant which is determined by the machine parameters. f.sub.k or .theta..sub.k is predetermined in a range where f.sub.k .ltoreq.f.sub.T or .theta..sub.k .ltoreq..theta..sub.T.
In the control system disclosed by the inventor's prior patent application, the control variable is generated in direct proportion to the position error in a range where the magnitude of the control variable is not larger than f.sub.k or .theta..sub.k. In a range where the magnitude of the position error is large and the magnitude of the initial value for the control variable as determined by the position error exceeds the predetermined limit, the predetermined limit f.sub.k or .theta..sub.k is used as the magnitude of the control variable. The direction of the control variable is determined by the direction of the position error.
Thus, by the disclosure of the inventor's prior patent application, the problem of the automatic control system of an alternating-current motor is solved. But, in practical applications, there are some disadvantages to be further improved in the control system of the inventor's prior patent application.
One of the disadvantages is that the system of the inventor's prior patent application is based on a principle of velocity-detection and frequency-synthesis. The measurement of a velocity naturally takes a finite time and the time lag caused by this finite time is not desirable in some applications, especially in a control of a synchronous motor.
Other disadvantages come from performance characteristics of the inverter included in the control system. The control variable of the inventor's prior patent application can be either positive or negative in the limited range of the magnitude, generating either a positive (an accelerating) torque or a negative (braking) torque. For a negative value of the torque to be generated, an electric current will flow from the motor to the inverter when the velocity of the motor is not sufficiently small. Some of the conventional type static inverters do not absorb the power generated by the motor. Therefore, when the control variable is negative in a range where the motor velocity is not sufficiently small, this negative power input to the motor (negative power output from the inverter) must be properly controlled.
Although a unity power-factor is desirable for some type of static inverters, the power factor of a synchronous motor is improved only by the control of the field exciting current in the control system of the inventor's prior patent application. The power factor improvement obtained by controlling the amplitude V.sub.1 of the armature input voltage of a synchronous motor is more advantageous.
In one embodiment of the inventor's prior patent application, the frequency f.sub.1 of the power supply voltage is controlled in direct proportion to the position error in accordance with an equation f.sub.1 =G.sub.1 (.theta..sub.c -.theta..sub.a) where .theta..sub.c is the reference position, .theta..sub.a is the controlled position, and G.sub.1 is an amplification factor. Since the slip frequency f.sub.s is f.sub.s =f.sub.1 -f.sub.2, where f.sub.2 is the motor velocity in electrical revolutions per second, the foregoing equation is equivalent to an equation f.sub.s =G.sub.1 (.theta..sub.c -.theta..sub.a)-f.sub.2, which means that a speed feedback term f.sub.2 is introduced in the control variable f.sub.2 which is nearly proportional to the torque T generated. This introduction of a speed feedback term is advantageous to stabilize the performance of the positioning control system.
But the disadvantage of this method for introducing the speed feedback term is that the coefficient of the speed feedback term is not adjustable.
In a velocity control of an induction motor according to claim 20 of the inventor's prior patent application, a generator means for generating a frequency corresponding to the reference velocity is included in the means for detecting the position error. This frequency generator means will be expensive when an adjustable reference velocity is required. This is one of the disadvantages to be improved in the control system of the inventor's prior patent application.